Grate Inlet in Sag
Grate Inlet
The flow Qiw intercepted by a grate inlet operating as a weir is:
| W | = | Width of the grate (m, ft) | |
| L | = | Length of the grate (m, ft) | |
| C W | = | Weir Coefficient (1.66 SI, 3.0 U.S. customary) | |
| d1 | = | Flow depth at middle of grate(m, ft) | |
| d 2 | = | Flow depth at side of grate opposite the curb (m, ft) |
The flow Qio intercepted by a grate inlet operating as an orifice is:
| Qio | = | Capacity of the grate operating as an orifice (cfs, m 3 /s) | |
| A | = | Clear opening area of grate (ft2 , m2 ) | |
| d | = | Average depth of flow over grate (ft, m) | |
| g | = | Acceleration due to gravity (ft/s2 , m/s2 ) |
The intercepted flow Qi is conservatively calculated at any flow depth by using the lesser of the intercepted flows computed using the weir or orifice equation:
This accounts for the three stages: weir flow, orifice flow and transitional flow.